The present invention relates to heavy duty shears of the type mountable on the boom of a mobile machine such as an excavator and, more particularly, to improvements in the hydraulics driving the shear.
Heavy duty shears of the type to which the present invention is directed are used in conjunction with the handling and shearing of a wide variety of materials including iron and steel pipes, steel structural shapes such as I-beams, channels and angles, steel encased in concrete, heavy miscellaneous shapes and the like. Such shears are used in scrap operations, demolition of factories and mills, construction and other industries. Such shears generally comprise pivotally interconnected upper and lower jaw members. The lower jaw member generally comprises a fixed cutting blade and a guide blade having a slot therebetween. The upper jaw comprises a movable blade which rotates around the pivot point into the slot to perform a shearing action on a workpiece in cooperation with the fixed cutting blade. One particularly successful such shear is described in U.S. Pat. No. 5,230,151 to Kunzman et al., which is incorporated herein by reference.
While such shears are operative for their intended purposes, they can jam. The shears are particularly subject to jamming when cutting steel plate and the like having a thickness appreciably less than the capacity of the shear. A piece of the steel plate can become lodged between the movable blade and the guide blade wedging the movable blade in the slot in the lower jaw. Generally, with heavy duty shears, the force available to close the shear is greater than the force available to open the shear. The wedging action can be so strongly driven that full available force to open the shear will not open the shear.
The difference in force available to close the jaws as compared to the force available to open the jaws is a function of the design of a hydraulically driven shear. A conventional hydraulic shear is driven by a conventional hydraulic cylinder. Such a cylinder contains a piston driven by hydraulic pressure connected to a rod which is in turn connected to the movable jaw of the shear. Pressure on the piston side of the piston acts on the entire surface of the piston and creates a closing force equal to the hydraulic pressure times u times the inside radius of the cylinder squared. The force available to open the jaws is generated by applying hydraulic pressure on the rod side of the piston. The area of the piston upon which the hydraulic pressure can work is diminished by the area occupied by the rod. Because the rod must have a substantial diameter to carry the large forces involved, it is not uncommon for the rod side of the piston to have only 50 or 75% the working area as the piston side.
The frictional forces involved in opening a jam are different from those encountered when the shear is moving. During normal shear operations, the shear blades and workpieces are moving and sliding with respect to one another. The amount of friction force resisting movement is the force normal (perpendicular) to the sliding surface times the coefficient of sliding friction. In a jam, motion has stopped. The amount of friction force resisting movement is the force normal to the surface times the coefficient of static friction. For almost all materials, the coefficient of static friction is larger than the coefficient of sliding friction. For dry hard steel on hard steel, the coefficient of sliding friction is 0.42 and the coefficient of static friction is 0.78. For dry mild steel on mild steel, the coefficient of sliding friction is 0.59 and the coefficient of static friction is 0.74. Coefficients for other materials are published in generally available hand books.
Because of the differences between sliding and static friction, more force is usually required to open a jam than to create it. For dry hard steel, approximately 1.8 to 1.9 more force will be required based solely on the different coefficients of friction. For dry mild steel, the different coefficients of friction require about 1.3 more force to break a jam.
The forces used to close and open heavy duty shears are enormous. Because such high forces are used in creating the jam, auxiliary mechanical means to clear the jam do not work. One cannot use a winch or hydraulic jack or the like to open the jaws because they have been closed with such enormous forces. Rather, the normal way of dealing with such jams is to cut out a portion of the workpiece and the shear itself with a cutting torch, replace or repair the cut part and return the shear to service. Considerable expense and downtime are consumed.